It is plain, then, that the human body is not helpless in the presence of the bacteria of disease, but that it is supplied with powerful resistant forces.  It must not be supposed, however, that the outline of the action of these forces just given is anything like a complete account of the matter; nor must it be inferred that the resistance is in all respects exactly as outlined.  The subject has only recently been an object of investigation, and we are as yet in the dark in regard to many of the facts.  The future may require us to modify to some extent even the brief outline which has been given.  But while we recognise this uncertainty in the details, we may be assured of the general facts.  The living body has some very efficacious resistant forces which prevent most bacteria from growing within its tissues, and which in large measure may be relied upon to drive out the true pathogenic bacteria.  These resistant forces are in part associated with the productions of body poisons, and are in part associated with the active powers of special cells which have been called phagocytes.  The origin of the poisons and the exact method of action of the phagocytes we may well leave to the future to explain.

These resisting powers of the body will vary with conditions.  It is evident that they are natural powers, and they will doubtless vary with the general condition of vigour of the individual.  Robust health, a body whose powers are strong, well nourished, and vigorous, will plainly furnish the conditions for the greatest resistance to bacterial diseases.  One whose bodily activities are weakened by poor nutrition can offer less resistance.  The question whether one shall suffer from a germ disease is not simply the question whether he shall be exposed, or even the question whether the bacteria shall find entrance into his body.  It is equally dependent upon whether he has the bodily vigour to produce alexines in proper quantity, or to summon the phagocytes in sufficient abundance and vigour to ward off the attack.  We may do much to prevent disease by sanitation, which aids in protecting the individual from attack; but we must not forget that the other half of the battle is of equal importance, and hence we must do all we can to strengthen the resisting forces of the organism.

Recovery from germ diseases.

These resisting forces are not always sufficient to drive off the invaders.  The organisms may retain their hold in the body for a time and eventually break down the resistance.  After this they may multiply unimpeded and take entire possession of the body.  As they become more numerous their poisonous products increase and begin to produce direct poisoning effects on the body.  The incubation period is over and the disease comes on.  The disease now runs its course.  It becomes commonly more and more severe until a crisis is reached.  Then, unless the poisoning is so severe that death occurs, the effects pass away and recovery takes place.